The effect of reactive school closure on community influenza-like illness counts in the state of Michigan during the 2009 H1N1 pandemic.

BACKGROUND: In sum, 559 Michigan schools were closed as a nonpharmaceutical intervention during the influenza A 2009 (H1N1) pandemic. METHODS: By linking the proportion of schools closed within a district to state influenza-like illness (ILI) surveillance data, we measured its effect on community levels of ILI. This analysis was centered by the peak week of ILI for each school district, and a negative binomial model compared three levels of school closure: 0%, 1%-50%, and 51%-100% of schools closed from three weeks leading up to ILI peak to four weeks following ILI peak rate. RESULTS: We observed that school closures were reactive, and there was no statistically significant difference between ILI rates over the study period. There was an elevated rate ratio for ILI at 51%-100% closure, and a reduction in the rate ratio at the 1%-50% compared to the 0% closure level. CONCLUSIONS: These findings suggest that district level reactive school closures were ineffective.